Process for detinning tinned sheet iron

ABSTRACT

This invention relates to a process of detinning tin-plated sheet iron with soda lye in the presence of sodium chlorite as an oxidizing agent.

United States Patent 1151 3,660,077 Ruf 1 51 May 2, 1972 [5 PROCESS FORDETINNING TINNED 1 References Cited SHEET IRON UNITED STATES PATENTSllh'mfiwmw 2,011,305 11/1935 Smith ..75 9s 73 Assign; I,3l(),38l 7/1919Wilcox ..75/98 2,082,487 6/1937 Frick ..75 98 [221 @1970 855,491 6/1907Acker.... ..75/98 211 AppI.No.: 11,290 872,092 11 1907 Sperry ..75/98Primary Examiner-Winston A. Douglas [30] Foreign Apphcatwn Pnomy DataAssistant Examiner-Peter D. Rosenberg Feb. 20, 1969 Germany ..P 19 08503.1 Attorney-James E. Bryan 52 us. c1 ..75 98, 75/64 57 ABSTRACT 5111111. c1. ..C22b 25/06 581 Field ofSearch ..75/64,85,98;23/89, 152relates 3 Pmcess demmmg sheet iron with soda lye in the presence ofsodium chlorite as an oxidizing agent.

3 Claims, No Drawings PROCESS FOR DETINNING TINNED SHEETIRON' Thisinvention relates to a process fordetinning tin-plated sheet iron, i.e.,tin-plate, with soda lye in the presenceof an" oxidizing agent.

In addition to the processing of tin ores, the recovery of'tin fromwaste tinned sheet iron plays an important role in'the production of tinmetal.

For the purpose of recovering tin from tin-plate, threepric. the removalof the tin with the aid of soda lye in the presence of a suitableoxidizing agent, with subsequent' elec trolytic separation of the tin.

Various compounds have been proposed as oxidizing. agents nitrite,alkali nitrate, alkali hypochlorite, alkali thiosulfate, and alkalipolysulfide. This process is preferably employed in the case of relativehigh tin-plate quantities.

In this process, only the tin which was applied to the iron galvanicallyor by hot-tinning as a superimposed layer during the tin-platingoperation is removed as a result of the reaction of the soda lye in thepresence of one of the aforementioned of sodium chlorite, if at leastequimolar quantities of sodium chlorite are present, tin is oxidizedfrom the nonvalenttstate through the bivalent to the tetravalent state,while sodium: chlorite is reduced to sodium chloride. Formed at the sametime is a soluble sodium hydroxo-stannatecomplex, but the iron is notattacked thereby and can be fumished, after detinning, to the steelindustry as a valuable scrap, so-called black plate. In this connection,it is of special significance that the residual tin contentof the blackplate is low if 'sodium chlorite is employed as theoxidizing agent.

It is a particular advantage resulting from the use of sodium chlorite,as proposed by the presentinvention, thatdetinning. can be effected inan economical manner atelevated temperatures if equimolar sodiumchlorite quantities, based on the tin, are present in excess quantitiesof soda lye. The excess of soda lye, however, may be maintained lowerthan was the case with the oxidizing agents used heretofore.

The detinning operation preferably is performed at an elevatedtemperature in the range of about 50 to l 'Cl Particularly preferred isthe temperature range of 80 to' 95 C.

When using excess quantities of soda lye and sodium chlorite, itgenerally suffices to perform the detinning operation with only onedetinning solution; An extensive detinning.

of the tinned sheet iron is effected by virtue of the effectivedetinning action in the presence of sodium chlorite.

Since, in the process of the invention, it is possible to employ low lyeconcentrations, relatively large quantities of tin may be removed inconformity with the relatively good solubility of tin in dilute sodalye, particularly if it is ensured that the soda lye and sodium chloriteconcentrations are continuously maintained in correspondence with theconsumption thereof. Actually, however, the concentration of soda lyealways should be so selected that excess quantities thereof are present,as compared to the lye concentration which is required ashydroxo-stannate for the complex formation of the tin.

Since the detinning of tin-plated sheet iron with soda lye in thepresence of sodium chlorite is particularly effective, detinning may beperformed in shorter periods of time than was possible with theoxidizing agents heretofore employed, employing the same processconditions in both cases.

Lacquered tin plate should be delacquered in known manner prior to thedetinning thereof with sodium chloritefor the removal of the tin withthe aid of soda "lye in the! presence of an oxidizing agent such as, forexample, .alkali containing soda lye .so that the sodiumchlorite-containing soda lye solution is fully efiective.

lt ispreferable that either the tin-plate to be detinned have arotarymovement imparted thereto in a-suitable vessel during thedetinningoperation or that theliquor be agitated and recycled. Thisensures an intimate contact of the detinning lye with the tin-plate. lfdesired,*air also. may be passed through the liquor from below thetin-plateito be detinned.

Since, during the detinning of tin-plate with sodiumchlorite-containing'soda' lye, sodium chloride is obtained as areduction product, it is desirable to separate the tin'contained in theliquor, by means of a subsequent precipitation process, in the 'form ofstannic acid and stannous hydroxide. The

separated stannic acid either may be subjected'to electrolysis,

after washing and dissolving in soda lye, with a view towardmetalrecovery in the form of the stannate solution thereby obtained or,with'a view toward therecovery-of solid sodium stannate, it may beemployed as sodium stannate solution for recoveringthe solid product. Inprinciple, theliquors obtained during the detinning of tin-plate withsodium chlorite-containing soda lye also may be directly fed to anelectrolysis operation but, in such case, a certain corrosion of theanode material. must be accepted due to the intermediate anodicformation of chlorine, and also of sodium hypochlorite and sodiumchlorate, depending upon-the temperature, as a result of an immediatesecondary reaction.

If sufficiently long detinning times are used, especially low residualtin contents may be achieved in the black plate obtained from theprocess.

The particularly good detinning of tin-plate with the aid of soda -.lyein the presence of sodium chlorite, as compared to heretofore knownoxidizing agents, is further illustrated by reference to the followingspecific examples.

EXAMPLE 1 Approximately l50 grams of tin-plated sheet iron, containing0.79-per cent by weight Sn, are placed into a cylindrical perforatediron drum, having a capacity of about 700 ml, which is rotatably mountedin an'iron tank having a capacity ofabout 4'liters. The ratio'of theperforated surface. to the metallic surface of the iron drum is 1 2 6.8.

The mounting of the perforated iron drum is so maintained thatabouttwo-thirds of the surface thereof is immersed (during the rotationthereof at the rate of 4.5 revolutions/min.) in the liquor (about '320ml) which is present in theiron tank.

The liquor, which contains 3' grams of NaOH/l and 0.32 gram NaClO ll, isheated, during the rotation of the perforated iron drum filledwithtin-plate, to about C. After COMPARATIVE EXPERIMENTS A. 150 grams oftin-plate, containing 0.79 per cent by weight Sn, are detinned, in theapparatus of Example 1, for 20 minutes at a temperature of 80 C. in aliquor containing 3 grams of NaOl-I/ 1 and 0.53 gram of NaOCl/l.

The detinned sheet iron (black plate) has a residual tin content'of 0.5l per cent by weight.

B. 150' grams of tin-plate, containing 0.79 per cent by weight Sn, aredetinned, in the apparatus of Example 1, for 20 minutes at a temperatureof C. in a liquor containing 5 gramsofNaOl-l/l and 2.0 grams of NaNO /l.

The detinned sheet iron (black plate) has a residual tin content of 0.30per cent by weight.

C. l50 grams of tin-plate, containing 0.79 per cent by weight Sn, aredetinned, in the apparatus'of Example 1, for 20 minutes at atemperatureof 100 C. in a liquor containing 5 grams of NaOl-l/l and 5 grams ofNaClO ll.

The detinned sheet iron (black plate) has a residual tin content of 0.40per cent by weight.

EXAMPLE 2 Approximately 200 kilograms of tin-plate, containing 0.74 percent by weight Sn, are introduced into a hexagonal perforated iron drum,having a capacity of about 1.5 m mounted in an iron tank having acapacity of about 4 m The ratio of perforated surface to metallicsurface of the hexagonal perforated iron drum is l 8 .5.

A detinning liquor, which contains 3.4 grams of NaOH/] and 032 gram ofNaClOdl, is heated to 85 C. during rotation, at one revolution/min, ofthe perforated iron drum filled with the tin-plate.

During the rotation of the drum, about half of the drum is immersed inthe liquor.

While air is simultaneously introduced below the drum, the detinning ofthe tin-plate is effected during 2 hours at a temperature of 85 C. Thedetinned sheet iron (black plate) displays a residual tin content of0.048 per cent by weight.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

1. In the process of detinning tin-plated sheet iron with excess sodalye, based upon tin, and in the presence of an oxidizing agent, theimprovement which comprises employing sodium chlorite (NaClO as theoxidizing agent, the sodium chlorite being present in at least equimolarquantities with respect to tin.

2. A process according to claim 1 performed at a temperature in therange of about 50 to 200 C.

3. A process according to claim 2 in which air is passed through thereaction mixture.

l I t i

2. A process according to claim 1 performed at a temperature in therange of about 50* to 200* C.
 3. A process according to claim 2 in whichair is passed through the reaction mixture.